Tetrathiafulvalene-Based Architectures: From Guests Recognition to Self-Assembly

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Tetrathiafulvalene-Based Architectures:

From Guests Recognition to Self- Assembly

Marc Sallé â , Dr David Canevet â , Dr Jean-Yves Balandier â , Dr Joël Lyskawa â , Dr Gaëlle Trippé â , Dr Sébastien Goeb â & Dr Franck Le Derf â

a Laboratoire MOLTECH-Anjou, UMR CNRS 6200, Université d’Angers, Angers, France

Available online: 12 Jul 2011

To cite this article: Marc Sallé, Dr David Canevet, Dr Jean-Yves Balandier, Dr Joël Lyskawa, Dr Gaëlle Trippé, Dr Sébastien Goeb & Dr Franck Le Derf (2011): Tetrathiafulvalene-Based Architectures: From Guests Recognition to Self-Assembly, Phosphorus, Sulfur, and Silicon and the Related Elements, 186:5, 1153-1168

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Phosphorus, Sulfur, and Silicon, 186:1153–1168, 2011 Copyright^C Taylor & Francis Group, LLC

ISSN: 1042-6507 print / 1563-5325 online DOI: 10.1080/10426507.2010.543099

TETRATHIAFULVALENE-BASED ARCHITECTURES:

FROM GUESTS RECOGNITION TO SELF-ASSEMBLY

Marc Sallé, David Canevet, Jean-Yves Balandier, Joël Lyskawa, Gaëlle Trippé, Sébastien Goeb, and Franck Le Derf

Laboratoire MOLTECH-Anjou, UMR CNRS 6200, Universit´e d’Angers, Angers, France

Abstract The tetrathiafulvalene (TTF) unit has been successfully used for an incredibly broad range of applications. Beyond the well-established conducting properties of the corresponding cation-radical salts, this unit has appeared as a key redox-active component for various applications supported by its remarkable redox properties: a highπ-donating ability and occurrence of three stable redox states. This article reviews the main contribution of the group of Angers to this field, highlighting results obtained in terms of redox-sensing as well as efforts carried out to reach new self-assembled TTF-based architectures.

Keywords Calixarene; metallacycle; organogel; redox-responsive ligand; self-assembly;

tetrathiafulvalene

INTRODUCTION

The tetrathiafulvalene (TTF, 1) unit is fascinating by the fact that it combines a simple S-rich heterocyclic structure with an incredibly broad range of applications.^1–5In addition to the tremendous efforts that have been produced for four decades in designing new TTF derivatives programmed to improve the solid-state electroconducting properties of their oxidized salts,^6,7important developments have also been carried out for preparing molecular materials presenting various physical properties, such as magnetic,⁸ NLO,⁹ or photovoltaic¹⁰ properties for example.^1–5,6,11In the recent period, the remarkable π- donating solution properties of this unit have also been explored in a large number of redox- switchable molecular and supramolecular architectures, which we recently reviewed.¹²The ability of TTF to support such switchable processes comes from remarkable electronic properties, manifested by a highπ-donating ability (low first oxidation potential E1ox) and the occurrence of two successive reversible one-electron redox steps (E¹1/2, E²1/2), which give rise to three stable redox states (TTF, TTF^+•, TTF²⁺) (Scheme 1). This review will highlight recent illustrative efforts from our group in the preparation of TTF-based systems, regarding (1) the preparation of redox-responsive ligands, including ones incorporating

Received 21 October 2010; accepted 23 November 2010.

All people from Angers and outside who have contributed to the results presented through various collabo- rations are warmly acknowledged. We thank the ANR PNANO “TTF-Based Nanomat” for financial support.

Address correspondence to Marc Sall´e, Laboratoire MOLTECH-Anjou, UMR CNRS 6200, Universit´e d’Angers, 2Bd Lavoisier, 49045 Angers Cedex, France. E-mail: marc.salle@univ-angers.fr

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calixarene-based scaffold, as well as (2) the use of self-assembly to produce electroactive organogels or metal-driven self-assembled molecular electroactive polygons.

Scheme 1 The three redox states of TTF1.

RESULTS AND DISCUSSION

TTF-Based Redox-Responsive Ligands

The well-defined redox and electron-donating properties of TTF have positioned this unit as a key redox building block in the construction of redox-sensors for cationic guests.^12–15 Pioneering work in this area using the TTF framework was performed in Odense, in the group of J. Becher and colleagues.^16,17Corresponding receptors are built from the covalent association of a binding subunit to an electroactive TTF derivative. A structural constraint for designing such assemblies lies in the need to ensure an efficient through-space or through-bond communication between both binding and redox subunits.

On this basis, sensing of a given guest that has been trapped or even modulation of the host–guest binding constant can be expected. In most cases, such systems work on the basis of electrostatic interactions between the guest and the redox unit; binding a cation in the cavity close to the TTF core leads to an alteration of theπ-donating ability of the latter. This behavior is ascribed to the inductive effect exerted by the bound-cation, and is generally easily monitored by cyclic voltammetry (CV), with typically a positive shift in the first redox potential (E¹_1/2) of the TTF subunit being seen. The synthetic methodology, initially developed in Odense,¹⁸and using selective thiolate protection/deprotection sequences by use of bromopropionitrile and cesium hydroxide monohydrate, was applied in our case to the synthesis ofbis-substituted TTF derivatives, which bear a binding unit lying along the long axis of the TTF skeleton. Crown-ether TTFs2^19,20and their sulfur analogue3²¹ (Scheme 2) exhibit high affinity for various targeted cations (typical K^◦ values 10³–10⁴ M⁻¹in CH3CN/CH2Cl2) as shown by¹H NMR, UV-vis, and/or by MS titration studies.

The recognition process could also be electrochemically monitored in the presence of stoichiometric (1:1) quantities of added metal cations. In particular, significant positive shifts in the first redox potential could be observed (e.g.,E¹1/2= +45–100 mV for2or3) (Figure 1). As expected from the hard/soft nature of interaction sites and from the size of the cavity, metal ion selectivity could be demonstrated as shown with2avs.2bor2avs.3;²¹ whereas the crown-ether systems2aand2bdemonstrate a good affinity for group II metal ions (Sr²⁺and Ba²⁺, respectively), receptor3 involving S-atoms binds preferentially the relatively soft cation Ag⁺, as well as do4and5with N-atoms (Scheme 2).^22,23Interestingly, in all cases, no variation in the second redox potential (E²_1/2) is observed upon introduction of the cation in the electrolytic solution. This is ascribed to the expulsion of the metal ion at this potential. A similar behavior was observed with compound6(Scheme 2),²⁴constructed around the bis(pyrrolo)TTF skeleton. In this case, a high affinity for Pb²⁺(log K^◦=6.2 in CH₂Cl₂/CH₃CN, 1/1), along with a remarkable electrochemical signature were found (E¹1/2= +140 mV).

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TETRATHIAFULVALENE-BASED ARCHITECTURES 1155

Scheme 2 TTF-based crown-ether derivatives2–6.

Figure 1 Cyclic voltammograms of compound2bin the presence of increasing amounts of Ba(ClO4)2(from 0.0 to 1.0 equiv.). (Color figure avialable online.)

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Figure 2 Cyclic voltammograms of compound7in the presence of increasing amounts of Pb(ClO4)2(from 0.0 to 1.2 equiv.). (Color figure available online.)

Receptors involving an acyclic binding site are of interest since they allow a more straightforward synthetic approach (no need for high-dilution conditions), but also present higher solubility and greater conformational flexibility than for crown-systems. The TTF- based podand7^25,26shows a good affinity for Pb²⁺(log K^◦=5.0, CH2Cl2/CH3CN (1/1)).

In this case, the progressive addition of Pb(ClO4)2 not only led to an anodic shift in the potentials, but also to the appearance of a new redox system (E¹1/2 = +120 mV) corresponding to formation of the7-Pb²⁺(1/1) complex (Scheme 3) at the expense of the free ligand7(Figure 2). Interestingly, the E²1/2values remain constant, a finding that once again is interpreted in terms of cation expulsion taking place in this oxidized form. The binding constants for the different redox states of TTF could be evaluated as log K^◦=5.5, log K^.⁺=3.3 and log K²⁺=0, respectively. Such behavior could be exploited in the design of devices able to electrochemically control metal binding via external adjustments in the redox state of the constituent TTF subunits.²⁵

Scheme 3 Molecular structure of acyclic receptors7–9.

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Figure 3 Deconvoluted cyclic voltammogram of compound9in the presence of increasing amounts of Cd(ClO4)2

(from 0.0 to 2.0 equiv.).

A considerable variety of TTF-pyridine derivatives have been described in recent years. We recently synthesized the TTF-pyridine ligand8,²⁷which involves an imine linker (Scheme 3). This system shows a good sensing ability to lead(II). This aptitude clearly follows a different mechanism than the one described above for TTF-based receptors such as 2–6, which is supported by electrostatic interactions. Indeed, chemosensor 8exhibits a dual responsive behavior to metal binding, with both colorimetric (color change from yellow to purple) and electrochemical (positive shift of E¹1/2) detections. These changes result from a strong modification of the intramolecular charge transfer, which takes place from the HOMO of the donating part (TTF) to the accepting pyridyl moiety upon metal coordination. Therefore, the recognition is based in this case on a through-bond interaction rather than on a through-space electrostatic interaction between the bound cation and the TTF moiety. Compound9(Scheme 3), which also incorporates pyridyl units, shows a high affinity for M(II) transition metal cations.²⁸In the particular case of Cd²⁺, this recognition ability is manifested by a remarkable four-wave voltammetric response (Figure 3), which reflects the strong stability of the metal complex in both oxidized states of TTF. This behavior is ascribed to the peculiar rigid spatial arrangement observed in the X-ray structure, where the metal appears locked in a strongly rigid environment and centered over the redox unit.

Case of calixarene-TTF assemblies. The calix[4]arene moiety is known in the literature as a suitable scaffold for generating three-dimensional binding cavities capable of complexing various guests with high affinity. Modulating the guest binding properties as a function of TTF redox state requires first a high binding constant (K^◦) for the neutral state. In this context, we synthesized TTF-based receptors10a,b and studied their metal ion binding ability (Scheme 4).^29,30 The cone structure of both calix[4]arene platforms 10a and10b promotes the construction of two 3D pockets. Binding studies performed with these receptors revealed a good affinity for sodium cation. A slight increase in the E¹_1/2 value [(E¹_1/2)= +30mV] could be observed upon addition of NaClO₄. Based on a similar concept, we recently studied in collaboration with B.-T. Zhao and colleagues

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thiacalix[4]arene analogues11(Scheme 4).³¹ These bisthiacalix[4]arenes tethered by an electroactive TTF unit exhibit an electrochemical response upon introduction of Ag⁺.

Scheme 4 Bis(thiacalix[4]arene) TTF receptors10and11.

The electroactive TTF core can also be used to contribute to anion recognition in redox-responsive ligands. The calixarene derivative 12(Scheme 5), which encompasses two secondary amide H-bond donating sites, shows a good affinity for H2PO4−and a cone structure that rigidifies upon anion binding.^32,33This conclusion was supported by¹H NMR spectroscopic data, as well as from the electrochemical response, wherein a negative shift in the first redox potential is observed in this case upon introduction of H2PO4−.

Using the calix[4]arene scaffold, we also prepared derivatives13³⁴and14,³⁵which contain two and four pendant TTF moieties, respectively, at the lower rim of the cone (Scheme 6). A tetracarbonyl binding cavity, which is known to exhibit a strong affinity for alkali cations, is generated in both cases through ester and/or amide groups. This cavity is appropriate for Na⁺complexation, as shown by X-ray diffraction studies, which reveal that the metal cation is coordinated by eight oxygen atoms in both cases. The CV titration with Na⁺ shows the expected positive potential shift of the first redox potential of TTF (E¹_1/2), and in addition is accompanied by a peculiar CV behavior manifested by a thinner redox wave when Na⁺is bound (Figure 4). This is assigned to a molecular conformation change upon complexation, from a flexible free ligand wherein the TTF units can interact at E¹1/2, to a more rigid form upon sodium complexation, for which TTF units cannot interact anymore, and resulting in independent redox behavior. Systems13,14thus provide examples of redox-triggered molecular movement involving an additional input through metal cation binding.

Case of biphenyl-TTF assemblies. Moving to a biphenyl scaffold, C2- symmetric TTF-derivatives such as 15 were prepared in collaboration with G. Delogu and coworkers (Scheme 7),³⁶ and show an original electrochemical recognition process upon binding of Pb²⁺, correlated to conformational changes occurring upon metal cation complexation. As in the previous examples (calixarene derivatives13,14), the interaction between TTF moieties can be modulated upon binding of a metal cation (Pb²⁺), due in this

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Scheme 5 Structure of anion receptor12.

case to a rotation around the Ar–Ar bond; such architectures constitute therefore interesting models to study long-distance conformational effects monitored electrochemically.

Case of π-extended-TTF derivatives. Efforts to new TTF-based responsive ligands have also been carried out through a design optimization of the redox-active unit.

Examples were recently proposed with two conjugated analogues TTF16³⁷(in collaboration with S. X. Liu and S. Decurtins) and17³⁸(in collaboration with P. Fr`ere) presenting a furano- quinonoid or a thienylene spacer between the 1,3-dithiol-2-ylidene rings (Scheme 8). Group

Figure 4 Deconvoluted cyclic voltammogram of compound13in the presence of increasing amounts of NaClO4

(from 0.0 to 1.0 equiv.). (Color figure available online.)

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Scheme 6 Molecular structure of receptors13and14.

Scheme 7 Biphenyl-TTF receptor15.

Scheme 8 Receptors incorporating extended TTF derivatives16and17.

I and II cations are efficiently detected by17(Na⁺or Ba²⁺:(E¹1/2)= +90 mV), according to a process very similar to that for parent TTF derivatives2–6. In the case of16, various M(II) cations could be recognized through pyridyl coordination. In particular, introduction of Pb²⁺, Pd²⁺, Ni²⁺was accompanied by a remarkable four-wave behavior, with both redox waves considerably positively shifted [ca(E¹_1/2)= +150 mV;(E²_1/2)= +140 mV].

Such original behavior illustrates the high stability of the complex even at the dicationic stage, and accounts for the increased charge delocalization and the lowered coulombic repulsion within oxidized states of such a conjugated system.

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Immobilization on surfaces. Considering practical aspects of chemical sensors and devices, a critical next step concerns the possibility to immobilize the functional active group (receptor) onto a solid surface, in particular to allow an easier separation between the host and the guest substrate. For this to work, the recognition process under interfacial conditions must mirror that seen in homogeneous solution. We achieved immobilization of the above TTF-based receptors either by preparing self-assembled monolayers (SAMs), by electrodeposition of persubstituted dendrimers, or by preparation of substituted electroconducting films. In the first case, SAMs were prepared from receptors2band7substituted with alkylthiol grafting sites.^39,40The corresponding monolayers demonstrated sufficient stability on the gold substrate as well as good sensing abilities for Ba²⁺and Pb²⁺cations.

Compound 2b was also immobilized onto a platinum surface through electrodeposition of dendrimeric molecules persubstituted on their periphery with up to 96 TTF units (collaboration with J.-P. Majoral, A.-M. Caminade et al.).⁴¹The resulting modified electrodes presented a nice electrochemical response to Ba²⁺. Finally, thick films of receptors2b⁴² and7²⁵ could be obtained on a conducting surface by electropolymerization. They were prepared from the redox-active receptor attached to a poly(ethylenedioxythiophene) (PE- DOT) or to a polypyrrole skeleton. In the latter case, the corresponding modified electrode allows for tuning of the binding/expulsion process for a given cation.²⁵In this context, and in line with the post-polymerization functionalization process that we previously described from the pentaethyleneglycol functionalized EDOT monomer18a,⁴³we recently demonstrated the efficiency of the new functionalized EDOT derivative18b(Scheme 9), bearing a highly nucleophilic masked thiolate function for generating various functionalized PEDOT- based films, either through direct electropolymerization or through a post-polymerization functionalization process.⁴⁴

Scheme 9 Synthetically versatile EDOT monomers18a,b.

A TTF-Based Organogel

Supramolecular polymers based on the TTF unit are also of interest, in particular to prepare organic conducting nanostructures like wires. Thus, a new approach consisting in designing TTF-based organogelators so as to prepare new conducting materials has been recently developed.^45–47Indeed, upon evaporation of the solvent from an organogel, a net- work of nanofibers—i.e., a xerogel—is generated and can be converted into a conducting material upon oxidation. Such nanowires are of utmost interest in the context of minia- turization and more generally, in nanotechnologies.⁴⁸In this regard, our contribution, in collaboration with the group of D. B. Amabilino, consisted in preparing and studying a mul- tifunctional gelating TTF-pyrene conjugate19in which various intermolecular interactions are competing, namely S. . .S contacts, H-bonding, hydrophobic, andπ−π interactions

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Figure 5 Aggregates obtained by cooling a hot solution of19(9.1×10⁻⁴mol.L⁻¹) in acetone observed by polarized optical microscopy (left); xerogel obtained after THF evaporation observed by SEM (right).

(Scheme 10).⁴⁹As illustrated in Figure 5, this structural peculiarity allowed the obtention of (i) varied nanostructures from bad solvents (e.g., acetone and dioxane) and (ii) organogels in miscellaneous solvents (e.g., tetrahydrofuran and chlorobenzene).

Scheme 10 Molecular structure of TTF-pyrene organogelator19.

The corresponding xerogels were subsequently doped to the radical-cation state, af- fording conducting materials that were fully characterized by infrared absorption, electronic paramagnetic resonance, and current-sensing atomic force microscopy (CS-AFM). These studies enabled us to confirm robustness of the (weak rearrangement occurred upon iodine doping) as well as the efficiency of the doping process. Nicely, a correlation between the EPR signal line width and xerogel conductivity was observed in most solvents, showing that the larger the EPR signal, the lower the resistivity. This work opens a large avenue to stimuli-responsive organogels and xerogels given the structure of compound19, which consists of a redox-active unit linked to a versatile fluorophore (pyrene) through a bisamide bridge suitable for anion recognition.

Metal-Driven Self-Assembly of TTF-Based Components

The coordination-driven self-assembly methodology has proven to be of particular interest for preparing highly organized architectures, including molecular polygons and

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polyhedrons.^50–56This methodology constitutes an efficient alternative to classical covalent multistep synthesis for preparing structures that are otherwise very challenging to reach.

Use of this strategy needs the previous synthesis of functional ligands (e.g., based on the pyridyl unit), which are then associated to complementary metallic salts of defined geometries (e.g., square planar Pt(II) and Pd(II) salts). Coordination results in metal- assembled entities of controlled shape. Besides the synthetic challenge, the resulting host architectures, which possess predefined geometries, present suitable characteristics in terms of molecular recognition.

The π-donating ability of tetrathiafulvalene (TTF) derivatives is well-established (vide supra), and has projected this unit as a key redox building block for the construction of various redox-switchable ligands.¹² On this basis, we recently used the metal driven self-assembly technique to reach a new class of receptors incorporating two⁵⁷or four⁵⁸ TTF-based redox units in a predefined geometry, through coordination to Pd(II) or Pt(II) phosphine corners. Whereas one pyridyl moiety allows preparation of tweezers-like structures, two pyridyl units are required on the periphery of the TTF backbone to prepare molecular polygons. Several TTF derivatives bearing two pyridyl coordinating moieties are described.^27,59–61 Nevertheless, considering the propensity of bis-substituted TTF derivatives to undergo aZ/Eisomerization upon oxidation to TTF^+•or by protonation,^20,62,63we have instead focused our interest on the non-isomerizable N,N-disubstituted derivatives of bis(pyrrolo)tetrathiafulvalene (BPTTF). The synthesis of BPTTF, a unit that occupies a growing place among electroactive supramolecular architectures,⁶⁴involves a cycloaddition between ethylenetrithiocarbonate and an electrophilic alkyne as the initial step. In the original synthesis by the group of J. Becher and colleagues,⁶⁵ the cycloaddition is carried out using dimethyl acetylenedicarboxylate as the electron-poor alkyne. We proposed recently⁶⁶an alternative straightforward synthetic route to BPTTF using acetylene dicarbaldehyde mono(diethyl)acetal20(scheme 11).^67,68The choice of this alkyne avoids a subsequent oxidation step along the synthetic pathway. The key step in this route relies on the treatment of amine intermediate22by Amberlyst 15. Three successive elementary reactions are involved along this step: acetal deprotection, ring closure, and aromatization.

Using this procedure, the N-tosyl-(1,3)-dithiolo[4,5-c]pyrrol-2-one23, a key intermediate to bispyrroloTTF (BPTTF) derivatives could be obtained in less than three days, in a global yield of 51% from acetylene dicarbaldehyde mono(diethyl)acetal20(Scheme 11).

Scheme 11 Synthetic route to N-tosyl-(1,3)-dithiolo[4,5-c]pyrrol-2-one23, a precursor of BPTTF.

Using a similar methodology, we recently proposed⁵⁷ a general access to N-aryl- 1,3-dithiolo[4,5-c]pyrrole-2-thione derivatives, key building blocks to N-aryl pyrroloTTF derivatives. In particular, this strategy allowed preparation of the N-pyridyl derivatives

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25a,b, which were coupled in refluxing triethylphosphite, to afford the target mono- and bispyridyl ligands26and27, respectively (Scheme 12).

Scheme 12 Synthesis of the mono- and dipyridyl BPTTF derivatives26and27.

After obtaining the monotopic and ditopic ligands26and27, we could proceed to their metal-directed self-assembly through coordination tocis-blocked square planar Pt(II) and Pd(II) complexes.⁵⁷ In the case of 26, rigid and orthogonal MPTTF dimers such as 28were obtained, featuring electron-rich tweezers-like structures (Scheme 13). The two redox-active units behave independently, showing no detectable intramolecular interaction by cyclic voltammetry and by thin layer cyclic voltammetry studies. Interestingly, the orthogonal electron-rich dimer28presents a good affinity for the electron accepting C60

[log K=3.15 (CH2Cl2) with a 1/1 stoichiometry] as checked by UV-vis titration studies.

It is worth noting that high third-order nonlinear optical responses could be found from these orthogonal dimers.⁶⁹In addition, such corner systems constitute interesting models for the construction of higher symmetrical metalla-assembled systems such as molecular squares.

Scheme 13 Synthesis of dimer28.

The latter could be obtained from treatment of ditopic ligand27with a cis-blocked dpppPt(II) complex, which produces polygon29(Scheme 14).⁵⁸To the best of our knowl- edge, this functional metallosupramolecular square is the first described that incorporates strongπ-donating side-walls. The redox behavior of this assembly presents the usual fea- ture of BPTTF derivatives, with two successive reversible redox processes. Therefore this

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molecular square allows a measure of control over the global charge surrounding the inter- nal cavity, since each of the four constituting BPTTF redox-units can be oxidized twice.

This renders this self-assembled square particularly attractive as a very easily oxidizable π-donating macrocyclic model, complementary to the extensively studied reducible ones generally based on the bipyridinium unit.

Scheme 14 Synthesis of the redox-active molecular-square29.

CONCLUSIONS

The unique electronic characteristics of TTF and its derivatives have installed this unit as a key building block for various applications in recent issues of molecular and supramolecular chemistry.¹²Through selected examples of our research activity in Angers, we have illustrated in this review this versatility, focusing on as different topics as redox- responsive ligands, self-assembled redox-active organogels, and construction of metal- driven self-assembled electroactive molecular polygons. Beyond those examples, and in addition to the development of new conducting materials, which is still very active,⁶TTF and its derivatives still deserve further developments in various fields. There is no doubt that new self-assemblies of TTF-containing functional molecules in solution into multilevel structures, as well as nanofibers deposited on surface for bottom-up assembly of functional supramolecular wires, will receive more and more attention in relation with molecular electronics, as will single molecules that function as switches on metal surfaces.^70–74 Regarding the tunable redox properties of TTF, switchable architectures engaged in sensing processes, such as molecular clips and tweezers,⁷⁵will certainly know new developments, in particular by taking benefit of the electron-rich character of this unit, well-suited for recognition of electron deficient guests.

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